This invention relates to a motor controller for controlling a brushless motor and an electric power steering system including the motor controller.
Generally, an electric power steering system for giving steering assist force to a steering mechanism of a vehicle by driving an electric motor in response to a steering torque added to a steering wheel by a driver has been used. A brush motor has been widely used as the electric motor of the electric power steering system; in recent years, a brushless motor has also been used from the viewpoints of improvement of reliability and durability, a decrease in inertia, and the like.
Generally, to control torque generated by a motor, a motor controller detects a current flowing into the motor and performs PI control (proportional-plus-integral control) based on the difference between the current to be supplied to the motor and the detected current. A motor controller for driving a three-phase brushless motor is often provided with two or three current sensors to detect current of two phases or more.
However, the number of the current sensors can be reduced to one. While the brushless motor rotates, the current value detected in each current sensor changes in response to a PWM signal. That is, within one period of the PWM signal, the current sensor may detect drive current of one phase and may detect the sum of drive currents of two phases. Then, since the sum of drive currents of three phases becomes zero, the remaining one-phase drive current can be found based on the sum of drive currents of two phases.
Relating to the invention, Japanese Patent Laid-Open No. 2007-118823 discloses a configuration for estimating the rotation angle when a rotation angle sensor of a motor is abnormal. Japanese Patent Laid-Open No. H10-167086 discloses a configuration for performing open loop control when current detection is abnormal. Further, Japanese Patent Laid-Open No. 2006-131191 discloses a configuration for using a map of an assist gradient in an assist map. Further, Japanese Patent Laid-Open No. H10-167085 discloses a configuration in which a switch connected to a power supply section is implemented as an FET.
As described above, in the motor controller provided with only one current sensor, when drive currents of two phases are actually detected, the remaining one-phase drive current is found by calculation. Therefore, if a failure occurs in the remaining one phase and an overcurrent flows or no current flows, the failure in the phase cannot be detected.